The present invention relates to, a circuit for processing image signals such as television signals, and more particularly to image signal correction circuit including a field memory or a frame memory. The circuit further having a motion-adaptive Y and C signal separation function and a scanning line interpolation function having a delay time equal to or longer than a vertical scanning period of a television signal, and an image signal processor which receives as an image signal a standard signal conforming to an NTSC system and a non-standard signal not completely conforming to the NTSC system to thereby process these signals for high image quality.
In an NTSC system using a composite color television signal as an example, the color subcarrier is modulated with color difference signals and the resulting signal is multiplexed in a 2.1-4.2 MHz band portion of a 4.2-MHz video signal band in order to transmit the color difference signals. The color subcarrier frequency f.sub.sc and a horizontal scanning frequency fh have the following relationship EQU f.sub.sc =455/2fh.
The horizontal scanning frequency f.sub.h and the vertical scanning frequency fv have the following relationship EQU fh=525/2fv.
Therefore, the phases of the color subcarrier spaced by one frame period are reverse to each other. For a still picture, it is possible to separate a luminance signal and a chrominance signal using the sum of adjacent frames and the difference between the frames, respectively. Thus cross components such as cross color or cross luminance (hanging dots) can be virtually eliminated to thereby result in a high quality image.
However, if such interframe processing is performed on a moving picture, a double image would result, and cross components would be removed insufficiently to thereby cause hanging dots and hence the deterioration of the image quality.
Motion-adaptive processing would be conceivable in which the motion of an image is detected from the difference between signals spaced by one frame period, interframe processing is performed to separate the luminance signal and the chrominance signal by regarding the image as being still if the detected motion of the image is small while intrafield processing is performed by regarding the image as moving to thereby separate the luminance signal and the chrominance signal if the motion of the image is large.
There is a technique in which for an interlace scan performed in the NTSC system, scanning lines are interpolated, scan signals are converted to a non-interlace (progressive) scan and displayed on the television receiver side. If an interpolation scanning line signal is prepared using scanning line signals, one field before, output from a field memory, converted to a progressive scan signal and displayed, a line flicker generated at an edge of a horizontal line can be eliminated. Although the field interpolation greatly benefits a still image, it also greatly deteriorates a moving picture to thereby result in an undesirable comb tooth-like double image.
Japanese Patent Publication JP-A-59-40772 discloses motion-adaptive processing in which the motion of a picture is detected in accordance with an interframe difference signal, an intrafield interpolation is performed if the motion of the picture is small while an interpolated scanning line signal is prepared using a scanning line signal in a transmitted field if the motion of the image is large.
FIG. 1 shows a block diagram of a luminance signal separating means B and a luminance signal scanning line interpolation means C of an image signal correction circuit comprising a Y and C signal separating circuit and a scanning line correction circuit connected in series. FIG. 1 denotes an input terminal 1; an analog-to-digital converter 7 (hereinafter referred to as ADC) which converts a composite analog color television signal received at the input terminal 1 to a digital signal; a frame memory 8 which delays the signal from the ADC 7 by one frame period a line comb filter 9 which separates a luminance (Y) signal and a chrominance (C) signal in the same field from the output signal of the ADC 7 (hereinafter referred to also as "performs an intrafield Y and C signal separation"); a frame comb filter 10 which receives the input and output signals of the frame memory 8 and separates the Y and C signals from those signals (hereinafter referred to also as "performs an interframe Y and C signal separation"); a motion detector 11 which receives the input and output signals of the frame memory 8 and detects the motion of the picture; a first mixer 12 which mixes the output signals from the comb filters 9 and 10 and also receives the output signal of the motion detector 11 to control the mixture ratio of those output signals; a line memory 13 which delays the output signal of the first mixer 12 by 1 H (one horizontal scanning period); an adder 14 which receives and adds the input and output signals of the line memory 13; a coefficient circuit 15 which multiplies by a factor of 2 the output signal of the adder 14; a first field memory 16 which delays the output signal of the first mixer 12 by one field period; a second mixer 17 which mixes the output signal of the coefficient circuit 15 and the output signal of the first field memory 16 and receives the output signal of the motion detector 11 as a control signal; a double-speed converter 18 which receives the output signals of the first and second mixers 12 and 17 and converts these signals to a progressive scanning television signal; and a digital-to-analog converter 19 (hereinafter referred to as DAC) which converts the digital signal from the double-speed converter 18 to an analog signal; and an output terminal 43.
The frame comb filter 10 separates the Y and C signals for a still image using the correlation between signal portions spaced by one frame period by addition of and substraction between adjacent frames. The line comb filter 9 separates Y and C signals for a moving picture using the correlation between adjacent lines in the same field. The mixture ratio of signals in the first mixer 12 is controlled by a signal indicative of the magnitude of the motion of the picture detected by the motion detector 11. If the magnitude motion of the picture is smaller than a predetermined value, the output signal, of the frame comb filter 10 is selected and outputted. Otherwise, if the magnitude of the motion of the picture is larger than a predetermined value the output signal of the line comb filter 9 is mainly selected and output.
A scanning line interpolation will now be described. In the interlace scan employed in the NTSC system, a position where an interpolation scanning line is to be formed in a field is scanned by the signal one field before. Therefore, in a still picture, the scanning line signal one field before may be used intactly as an interpolation scanning line. As will be recalled, such an interpolation between fields would rather deteriorate a moving picture, so that an interpolation scanning line signal for the moving picture is obtained by averaging two successive lines in the same field in the particular example. The mixture ratio of the input signals to the second mixer 17 is controlled by the output signal of the motion detector 11. If the motion of the picture is smaller than a particular value, the output signal of the first field memory 16 is selected and outputted. Otherwise, if the magnitude of motion of the picture is than a predetermined valve, the output signal from the coefficient circuit 15 is selected and outputted.
The double-speed converter 18 receives the output signals of the first and second mixers 12 and 17 as the current scanning line signal and the interpolation scanning line signal, respectively, compresses the time-base of these signals to one half, selects the current scanning line and the interpolation scanning line alternately for each scanning line, and sequentially outputs the selected signal as a progressive scanning signal.
If a non-standard signal such as a VTR reproduction signal which does not strictly keep the standards of the standard color television signal is processed on the presumption that the non-standard signal is a standard signal in the conventional example, the image quality would rather be deteriorated.
The signal processing in the conventional example is as shown in FIGS. 2A and 2B where the axis of abscissas represents the timebase and the axis of ordinates represents a vertical side of the screen. An circle represents a scanning line signal, a S(n) represented by a double circle in a Mth field denotes a signal input to the ADC 7.
As shown in FIG. 2A, for a still picture, Y and C signals are separated by the calculation on S(n) and S(n -525) to provide the current scanning line signal. The interpolation scanning line signal comprises a one-field period delayed version of the current scanning line signal and is calculated from S(n -263) and S(n-788). Therefore, the center of gravity of the image in the direction of the timebase, which is defined by a position on the timebase which is occupied by a particular image constituted by the current scanning line and interpolation scanning line positioned in different dimensions on the timebase, is between (M -1)th and (M -2)th fields.
As shown in FIG. 2B, for a moving picture, the separation of Y and C signals is performed by the calculation on S(n) and S(n -1), and an interpolation scanning line signal is calculated by averaging one-delayed versions of S(n) and S(n -1). Since the calculations are all performed in the same field, the center of gravity of the image in the timebase direction is in the Mth field.
This conventional image signal correction circuit does not have measures for coping with an input non-standard signal such as a VTR reproduction image which does not strictly keep the standards of the standard color television signal. Since the correction circuit has no measures to handle still and moving picture signals having different centers of gravity in the timebase direction, the center of gravity of the still picture signal and the center of gravity of the moving picture signal are spaced by 1.5 fields on the timebase for a picture which may move or stop, so that the motion of the picture appears unnatural sometimes.